This invention relates to improved photovoltaic devices which provide enhanced short circuit currents and efficiencies. The present invention has particular applicability to photovoltaic devices formed from layers of amorphous semiconductor alloys. The photovoltaic devices of the present invention include incident radiation directing means for directing either a portion or substantially all of the incident radiation through the active region or regions, wherein the charge carriers are created, at angles sufficient to cause the directed radiation to be substantially confined within the devices. This provides multiple reflections of the directed light in the active regions of the devices in which they are employed. One advantage of this approach is that increased photon absorption and charge carrier generation in the active regions is possible, providing increased short circuit currents. Another advantage is that since the directed light passes through the active region of the improved devices at an angle, the active region or regions can be made thinner to reduce charge carrier recombination while maintaining efficient charge carrier generation. The invention while not being limited to any particular device configuration, has its most important application in making improved amorphous silicon alloy photovoltaic devices of the p-i-n configuration, either as single cells or multiple cells comprising a plurality of single cell units.
Silicon is the basis of the huge crystalline semiconductor industry and is the material which has produced expensive high efficiency (18 percent) crystalline solar cells for space applications. For terrestrial applications, the crystalline solar cells typically have much lower efficiencies, on the order of 12 percent or less. When crystalline semiconductor technology reached a commercial state, it became the foundation of the present huge semiconductor device manufacturing industry. This was due to the ability of the scientist to grow substantially defect-free germanium and, particularly, silicon crystals, and then turn them into extrinsic materials with p-type and n-type conductivity regions therein. This was accomplished by diffusing into such crystalline material parts per million of donor (n) or acceptor (p) dopant materials introduced as substitutional impurities into the substantially pure crystalline materials, to increase their electrical conductivity and to control their being either of a p or n conduction type. The fabrication processes for making p-n junction crystals involve extremely complex, time consuming, and expensive procedures. Thus, these crystalline materials useful in solar cells and current control devices are produced under very carefully controlled conditions by growing individual single silicon or germanium crystals, and when p-n junctions are required, by doping such single crystals with extremely small and critical amounts of dopants.
These crystal growing processes produce such relatively small crystals that solar cells require the assembly of many single crystals to encompass the desired area of only a single solar cell panel. The amount of energy necessary to make a solar cell in this process, the limitation caused by the size limitations of the silicon crystal, and the necessity to cut up and assemble such a crystalline material have all resulted in an impassable economic barrier to the large scale use of crystalline semiconductor solar cells for energy conversion. Further, crystalline silicon has an indirect optical edge which results in poor light absorption in the material. Because of the poor light absorption, crystalline solar cells have to be at least 50 microns thick to absorb the incident sunlight. Even if the single crystal material is replaced by polycrystalline silicon with cheaper production processes, the indirect optical edge is still maintained; hence the material thickness is not reduced. The polycrystalline materials also contain grain boundaries and other defect problems, which are ordinarily deleterious.
In summary, crystal silicon devices have fixed parameters which are not variable as desired, require large amounts of material, are only producible in relatively small areas and are expensive and time consuming to produce. The use of devices based upon amorphous silicon alloys can eliminate these crystal line silicon disadvantages. An amorphous silicon alloy has an optical absorption edge having properties similar to a direct gap semiconductor and only a material thickness of one micron or less is necessary to absorb the same amount of sunlight as the 50 micron thick crystalline silicon. Further, amorphous silicon alloys can be made faster, easier and in larger areas than can crystalline silicon.
Accordingly, a considerable effort has been made to develop processes for readily depositing amorphous semiconductor alloys or films, each of which can encompass relatively large areas, if desired, limited only by the size of the deposition equipment, and which could be readily doped to form p-type and n-type materials which p-n junction devices are to be made therefrom equivalent to those produced by their crystalline counterparts. For many years such work was substantially unproductive. Amorphous silicon or germanium (Group IV) films are normally four-fold coordinated and were found to have microvoids and dangling bonds and other defects which produce a high density of localized states in the energy gap thereof. The presence of a high density of localized states in the energy gap of amorphous silicon semiconductor films results in a low degree of photoconductivity and short carrier lifetime, making such films unsuitable for photoresponsive applications. Additionally, such films cannot be successfully doped or otherwise modified to shift the Fermi level close to the conduction or valence bands, making them unsuitable for making p-n junction for solar cell and current control device applications.
In an attempt to minimize the aforementioned problems involved with amorphous silicon (originally thought to be elemental), W. E. Spear and P. G. Le Comber of Carnegie Laboratory of Physics, University of Dundee, in Dundee, Scotland, did some work on "Substitutional Doping of Amorphous Silicon", as reported in a paper published in Solid State Communications, Vol. 17, pp. 1193-1196 (1975), toward the end of reducing the localized states in the energy gap in amorphous silicon to make the same approximate more closely intrinsic crystalline silicon and of substitutionally doping the amorphous materials with suitable classic dopants, as in doping crystalline materials, to make them extrinsic and of p or n conduction types.
The reduction of the localized states was accomplished by glow discharge deposition of amorphous silicon films wherein silane (SiH.sub.4) gas was passed through a reaction tube where the gas was decomposed by an r.f. glow discharge and deposited on a substrate at a substrate temperature of about 500.degree.-600.degree. K. (227.degree.-327.degree. C.). The material so deposited on the substrate was an intrinsic amorphous material consisting of silicon and hydrogen. To produce a doped amorphous material phosphine (PH.sub.3) gas for n-type conduction or diborane (B.sub.2 H.sub.6) gas for p-type conduction were premixed with the silane gas and passed through the glow discharge reaction tube and under the same operating conditions. The gaseous concentration of the dopants used was between about 5.times.10.sup.-6 and 10.sup.-2 parts per volume. The material so deposited was shown to be extrinsic and of n or p conduction type.
While it was not known by these researchers, it is now known by the work of others that the hydrogen in the silane combines at an optimum temperature with many of the dangling bonds of the silicon during the glow discharge deposition, to substantially reduce the density of the localized states in the energy gap toward the end of making the electronic properties of the amorphous material approximate more nearly those of the corresponding crystalline material.
The incorporation of hydrogen in the above method, however, has limitations based upon the fixed ratio of hydrogen to silicon in silane, and various Si:H bonding configurations which introduce new antibonding states. Therefore, there are basic limitations in reducing the density of localized states in these materials.
Greatly improved amorphous silicon alloys having significantly reduced concentrations of localized states in the energy gaps thereof and high quality electronic properties have been prepared by glow discharge, as fully described in U.S. Pat. No. 4,226,898, Amorphous Semiconductors Equivalent to Crystalline Semiconductors, issued in the names of Stanford R. Ovshinsky and Arun Madan on Oct. 7, 1980, and by vapor deposition as fully described in U.S. Pat. No. 4,217,374 to Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, under the same title. As disclosed in these patents, which are incorporated herein by reference, fluorine is introduced into the amorphous silicon semiconductor alloy to substantially reduce the density of localized states therein. Activated fluorine especially readily bonds to silicone in the amorphous body to substantially decrease the density of localized defect states therein, because the small size, high reactivity and specificity of chemical bonding of the fluorine atoms enables them to achieve a more defect-free amorphous silicon alloy. The fluorine bonds to the dangling bonds of the silicon and forms what is believed to be a predominantly ionic stable bond with flexible bonding angles, which results in a more stable and more efficient compensation or alteration than is formed by hydrogen and other compensating or altering agents. Fluorine also combines in a preferable manner with silicon and hydrogen, utilizing the hydrogen in a more desirable manner, since hydrogen has several bonding options. Without fluorine, hydrogen may not bond in a desirable manner in the material, causing extra defect status in the band gap as well as in the material itself. Therefore, fluorine is considered to be a more efficient compensating or altering element than hydrogen when employed alone or with hydrogen because of its high reactivity, specificity in chemical bonding, and high electronegativity.
As an example, compensation may be achieved with fluorine alone or in combination with hydrogen, with the addition(s) of these element(s) being in very small quantities (e.g., fractions of one atomic percent). However, the amounts of fluorine and hydrogen most desirably used are much greater than such small percentages, so as to form a silicon-hydrogen-fluorine alloy. Such alloying amounts of fluorine and hydrogen may, for example, be in the range of 1 to 5 percent or greater. It is believed that the alloy so formed has a lower density of defect states in the energy gap than that achieved by the mere neutralization of dangling bonds and similar defect states. Such larger amount of fluorine, in particular, is believed to participate substantially in a new structural configuration of an amorphous silicon-containing material and facilitates the addition of other alloying materials, such as germanium. Fluorine, in addition to its other characteristics mentioned herein, is believed to be an organizer of local structure in the silicon-containing alloy through inductive and ionic effects. It is believed that fluorine also influences the bonding of hydrogen by acting in a beneficial way to decrease the density of defect states which hydrogen contributes while acting as a density of states reducing element. The ionic role that fluorine plays in such an alloy is believed to be an important factor in terms of the nearest neighbor relationships.
Amorphous silicon alloys containing fluorine have thus demonstrated greatly improved characteristics for photovoltaic applications as compared to amorphous silicon alloys containing just hydrogen alone as a density of states reducing element. However, in order to realize the full advantage of these amorphous silicon alloys containing fluorine when used to form the active regions of photovoltaic devices, it is necessary to assure that the greatest possible portion of the available photons are absorbed therein for efficiently generating electron-hole pairs.
The foregoing is important in, for example, photovoltaic devices of the p-i-n configuration. Devices of this type have p and n-type doped layers on opposite sides of an active intrinsic layer, wherein the electron-hole pairs are generated. They establish a potential gradient across the device to facilitate the separation of the electrons and holes and also form contact layers to facilitate the collection of the electrons and holes as electrical current.
Not all of the available photons are absorbed by the active regions in a single pass therethrough. While almost all of the shorter wavelength photons are absorbed during the first pass, a large portion of the longer wavelength photons, for example, photons having wavelengths of 6,000 angstroms or greater, are not so absorbed. The loss of these unabsorbed photons places a limit on the short circuit currents which can be produced. To preclude the loss of these longer wavelength photons, back reflectors, formed from conductive metals have been employed to reflect the unused or unabsorbed light back into the active regions of the devices.
The p and n-type layers are conductive and, at least in the case of the p-type layer, can have a wide band gap to decrease photon absorption. A back reflector is therefore extremely advantageous when used in conjunction with a p-type layer having a wide band gap forming the top layer of such a device. Back reflectors are also advantageous when the wide band gap p-type layer forms the bottom layer of the device. In either case, back reflecting layers serve to reflect unused light back into the intrinsic region of the device to permit further utilization of the solar energy for generating additional electron-hole pairs. A back reflecting layer permits a greater portion of the available photons to pass into the active intrinsic layer and to be absorbed therein.
Unfortunately, the best back reflectors of the prior art have been capable of reflecting only about 80 percent of the unused light back into the devices in which they are employed. Metals such as copper and aluminum, because they are highly reflecting, have been suggested as possible back reflector materials. However, these metals can diffuse into the semiconductor of the devices in which they are employed and, in doing so, adversely affect the photoresponsive characteristics of the devices. As a result, other less reflective metals have been employed as back reflectors. Such less reflective metals include molybdenum and chromium. Although these metals do not diffuse into the semiconductor of the devices, they cannot achieve the reflectance of the more highly reflective metals. This is particularly true when the less reflective metals interface with a material such as amorphous silicon alloys which have a high index of refraction. Furthermore, the back reflectors of the prior art reflect the unused light back into the active regions in the same direction as the original direction of incidences (assuming normal incidence). Hence, after being reflected, the light which is not absorbed during the second pass is permitted to escape. Hence, not all the light is absorbed. Also, since the light passes normal to the active regions, the active regions must be of sufficient thickness to permit efficient absorption. However, because the minority carrier diffusion length is finite, the active region cannot be made arbitrarily thick. If, to achieve substantial absorption, the active region thickess is increased much beyond the diffusion length, recombination effects will predominate, making it difficult to efficiently collect the photogenerated charge carriers as electrical current. Hence, there is a need for better photovoltaic devices which not only provide greater utilization of the incident light, but also more efficient collection of the charge carriers created in the active region or regions of the devices.
Applicants herein have discovered new and improved photovoltaic devices which provide both increased light utilization for creating electron-hole pairs and more efficient collection of the charge carriers. Basically, the present invention provides means for directing at least a portion of the incident radiation through the active region or regions at an angle which is sufficient to confine the directed light within the devices to substantially increase absorption. Further, the present invention permits the active regions to be made thinner to reduce recombination effects. The radiation directors of the present invention can be utilized in any form of photovoltaic cell, and find particular application in thin film solar cells in both single cell photovoltaic devices of the p-i-n configuration, and multiple cell structures having a plurality of single cell units.